When evaporating or cooling a liquid or solution in a gaseous medium, the manner in which contact is made between the liquid and gas has been found to play prime importance in determining the efficiency of the processes. Conventionally, evaporating ponds containing a liquid have been used for many years to evaporate liquid or concentrate a solution, with solar radiation being used as the energy source of evaporation and the liquid evaporated being released in the form of vapors to the free atmosphere. This has been common practice in many industrial and agricultural processes used in industries such as the salt producing, industrial waste disposal, mineral recovery and potash producing industries. In such methods and devices, however, the contact between the liquid being evaporated and the gaseous medium into which vapors are released and the exposure of the liquid to the energy source is minimal since only the upper surface of the liquid present in the pond is exposed to the ambient air.
For example, when minerals are recovered using large-scale evaporating ponds, solar radiation absorbed by these ponds supplies latent heat of evaporation permitting the solution to become progressively concentrated. However, since the vapor pressure of the solution or brine, being small in comparison with fresh water at the same temperature, decreases with increasing concentration, evaporation is retarded as the solution concentration increases. Furthermore, as the temperature of the solution rises, the sensible and long-wave radiation flux from the pond increases to a level where it exceeds the latent heat flux that produces evaporation and brine concentration. Consequently, only a fraction of solar energy used in an evaporation pond is converted into useful latent heat flux.
On the other hand, when cooling a liquid, forced draft cooling towers and cooling spray ponds have conventionally been used in industrial processes such as in the condensers of electric power plants, etc. Also natural draft and atmospheric spray cooling towers have been used in several applications. Normally, pressure spray nozzles or nozzles which produce droplets with a spectrum of different droplet sizes are used to produce liquid droplets for exposing the liquid to the gaseous medium. Forced draft cooling towers have been found to consume considerable amounts of energy, mainly due to the fans used to supply air to the cooling towers while spray ponds have been found to suffer from, among other things, inefficient supply of air into the spray body, and a loss of the liquid from the system due to the drift of droplets in winds present to locations external to the spray pond area. Also, due to the fact that the droplets have a spectrum of different droplet sizes, much of the cooling potential or the liquid cannot be efficiently used.
In U.S. Pat. No. 4,704,189 of the present inventor, the disclosure of which is hereby incorporated by reference, a method of and apparatus for evaporating liquid from a solution is disclosed wherein a starting solution is concentrated to a desired end solution without the necessity of using external heat. This is achieved by contacting the starting solution with a gaseous medium under the conditions that the heat content of the starting solution in contact with the gaseous medium is smaller than the heat content of the gaseous medium and the duration of contact is such that most of the evaporation of liquid from the solution takes place under conditions of constant enthalpy. According to this patent, to obtain such conditions, the starting solution is exposed to the gaseous medium for a sufficient period of time by either spraying the solution into the gaseous medium using pressure nozzles, rotating discs or piezoelectric sprayers or by passing the gaseous medium through a matrix of filaments over which a thin film of the solution flows. When a spray is used, droplets of a radius not greater than 0.2 mm are preferred, with the spray nozzles being positioned such that the spray travel is in excess of 5 m in order to achieve the desired result. When spraying liquid into the gaseous medium in accordance with this patent, use of spray nozzles which produce droplets of different diameter would be very undesirable, since prevailing winds can cause the drift of many of the relatively small droplets out of the spray area and consequently bring about their loss from the system and the possible contamination of the surrounding environment. Furthermore, use of conventional pressure nozzles for producing the required droplet spray is undesirable since relatively high pressures, conventionally greater than several atmospheres, need to be used, bringing about a relatively high energy consumption and making an operating pressure of several tens of atmospheres necessary merely to double the quantity of liquid used. Also, since the operating pressure of such nozzles influences the size of the droplets produced, flexibility of operation is minimal.
It is therefore an object of the present invention to provide a new and improved method of and apparatus for evaporating and cooling a liquid wherein the disadvantages as outlined are reduced or substantially overcome.